統計最近6個月內不同類型訂單的成交金額

本文出處：https://www.cnblogs.com/zuidongfeng/p/9048618.html
1、創建表

CREATE TABLE `t_order` (
  `id` bigint(4) NOT NULL AUTO_INCREMENT COMMENT '主鍵，自增',  
  `source` smallint(1) NOT NULL COMMENT '訂單來源:1:商城 2:訂單導入',
  `status` smallint(1) NOT NULL COMMENT '訂單狀態:1: 待確認 2：已確認 3:已完成',
  `order_amt` decimal(20,8) NOT NULL COMMENT '訂單總金額',
  `create_user_id` bigint(4) DEFAULT NULL COMMENT '創建人id',
  `create_time` datetime DEFAULT NULL COMMENT '創建日期',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='訂單表';

2、插入數據

INSERT INTO `t_order` (`id`, `source`, `status`, `order_amt`, `create_user_id`, `create_time`) VALUES ('1', '1', '2', '8000.00000000', '1', '2018-01-01 22:12:41');
INSERT INTO `t_order` (`id`, `source`, `status`, `order_amt`, `create_user_id`, `create_time`) VALUES ('2', '2', '3', '9000.00000000', '1', '2018-05-16 22:13:24');
INSERT INTO `t_order` (`id`, `source`, `status`, `order_amt`, `create_user_id`, `create_time`) VALUES ('3', '1', '3', '5000.00000000', '1', '2018-01-17 22:14:00');
INSERT INTO `t_order` (`id`, `source`, `status`, `order_amt`, `create_user_id`, `create_time`) VALUES ('4', '1', '3', '12000.00000000', '2', '2017-11-16 22:15:03');
INSERT INTO `t_order` (`id`, `source`, `status`, `order_amt`, `create_user_id`, `create_time`) VALUES ('5', '2', '3', '7000.00000000', '2', '2018-04-18 22:15:43');

3、統計近6月不同類型訂單的成交額

SELECT
    six_month.`month` `month`,
    source.source source,
    IFNULL(SUM(o.order_amt), 0) orderAmt
FROM
    (
        SELECT DATE_FORMAT(now(), '%Y-%c') month FROM DUAL
        UNION ALL SELECT DATE_FORMAT(date_sub(now(), INTERVAL 1 MONTH), '%Y-%c') month FROM DUAL
        UNION ALL SELECT DATE_FORMAT(date_sub(now(), INTERVAL 2 MONTH), '%Y-%c') month FROM DUAL
        UNION ALL SELECT DATE_FORMAT(date_sub(now(), INTERVAL 3 MONTH), '%Y-%c') month FROM DUAL
        UNION ALL SELECT DATE_FORMAT(date_sub(now(), INTERVAL 4 MONTH), '%Y-%c') month FROM DUAL
        UNION ALL SELECT DATE_FORMAT(date_sub(now(), INTERVAL 5 MONTH), '%Y-%c') month FROM DUAL
    ) six_month
CROSS JOIN (
    SELECT 1 source FROM DUAL
    union all 
    SELECT 2 source FROM DUAL
) source
LEFT JOIN t_order o ON o.source = source.source
AND DATE_FORMAT(o.create_time, '%Y-%c') = six_month.`month`
AND `status` = 3
GROUP BY
    six_month.`month`,
    source.source;

結果如下

思路：其實就是三張表連接，six_month表，source表，t_order表
注意：在 MySQL 中（僅限於 MySQL） CROSS JOIN 與 INNER JOIN 的表現是一樣的，在不指定 ON 條件得到的結果都是笛卡爾積，反之取得兩個表完全匹配的結果。INNER JOIN 與 CROSS JOIN 可以省略 INNER 或 CROSS 關鍵字。